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FCD diode... hitting fuel cut


83stariontravis
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So I just got my 16g td05h turbo installed and set for 12psi and now I'm hitting fuel cut again. Red light on the 83 boost gauge lights up and it cuts. Before with the 12a at 10.5 psi I was fine after I added the diode... Going to look into it more tomorrow just wondered if anyone else had this issue.
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It isn't hitting cut very hard. Before the diode it hit really hard. I'm not remembering at the moment exactly how a zener diode specifically functions. Just that it shunts the signal to ground when it reaches 4.3v. Is it possible for the diode to lag as the the voltage reaches the threshold it takes a second for it to clamp the voltage. More likely maybe the diode is being required to shunt more current to ground than it can so the voltage is spiking. I think I may add another in parallel and see what that does. HKS FCD... maybe. No real info on how that should be connected on an 83. I think I might opt to just run megasquirt standalone fuel management(not a bad way to spend ten thousand dollars) before I did that.
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Instead of a single one watt diode I added two more wired in parallel with the first so now there are three diodes dividing the current between them. Two would have probably been enough I just wanted to make sure. Apparently the factory map sensor is probably capable of outputting more than one watt and overwhelming the diode at higher boost levels.
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Actually air fuel ratios are kinda irrelevant to this. Same before and after. 3 diodes in parallel or one diode the voltage is being limited to 4.3v to the ecu. Just divides the current between the three diodes to make sure the diodes can accurately shunt enough to keep the voltage steady at 4.3v when the map sensors output would normally exceed that.
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If you connect a zener diode between the map sensor output signal to the ecu, and ground, you're going to end up stuffing your map sensor. Once the signal voltage reaches 4.3v (or whatever value zener you're using), the zener will shunt all the current the map sensor can produce to ground, in order to maintain a voltage drop across its body of 4.3v (or whatever value the zener is). This will essentially look like a dead short to your map sensor at this point, and you might end up frying it. The way around this is to put a resistor in the line before the zener shunt, to limit the current the zener will pass. This assumes that the ECU has a really high input impedance, which is a pretty safe assumption.

 

Adding more zener diodes in parallel won't really help, as they won't be completely identical. One of them will reach its threshold voltage before the others, and begin passing current. Depending on the zener diodes you've used, and how sharp their knee point is, one of your diodes will still be doing almost all the work, whichever one has the threshold voltage marginally lower than the others.

 

For a 4.3v zener, you'd need to pass ~230mA of current through it for the zener to be dissapating 1W. The map sensor surely won't be designed to output this much current to the ECU, as the ECU would never require a signal drvier that strong. I'd be really cautious of stuffing your map sensor. Luckily its a simple check to make sure it's all still working. Remove you're zener shunts, and confirm the map sensor still outputs a nice clean signal to the ECU with a multimeter (or better yet an oscilloscope).

 

I've never heard of people having particularly successful results using FCD's with the earlier starions, it's almost as if the factory ECU uses some other metric to determine when to provide the safety cut.

 

Good luck!

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Cool. I had thought about the resistor idea when I had this issue because it came up before. Do you have a recommendation on what resistor I should use in series with the map sensor before it reaches the ecu/diode? Factory boost gauge shows the map output on the USDM 83s and it still seems consistent and linear so I am fairly confident that it is still outputting same as ever. I do have an o-scope so I could hook that up if needed or I am close to getting my wbo2 and data-logger installed and that would work as well.
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